Traditionally, four-cycle internal combustion engines have relied on a valve train having “poppet” or mushroom intake and exhaust valves to feed the combustible air fuel mixture into the cylinder(s), seal the cylinder(s) during combustion, and to expel the burned fuel air mixture. Numerous alternatives to poppet valves have been tried over the hundred plus years of internal combustion four cycle engine development (including, sleeve valves, rotary valves, and slide valves to name a few). However, the vast majority of today's engines still rely on poppet valves because of the valve's ability to provide excellent sealing at an economical cost. Although the present invention may be used in other applications, it is primarily directed at engines using poppet valves.
As indicated above, the valve train typically consists of valves and a camshaft to actuate the valves (by an opening and closing mechanism). The camshaft, typically a long round shaft, includes lobes shaped and ground into the shaft to create offset motion (lift). As the camshaft spins, the lobes open and close the intake and exhaust valves in a synchronized relationship with the motion of the piston. The camshaft can be located directly over the valves (overhead camshaft), generally between the intake and exhaust valves, in either a single or double camshaft arrangement (SOHC or DOHC). So, for example, in a SOHC engine, the engine will have one cam if the engine is an inline 4-cylinder or inline 6-cylinder. If instead the SOHC engine is a V-engine (for example, V-6 or V-8), it will have two cams (one for each cylinder head), even though each is a “single” overhead camshaft. Similarly, DOHC engines have two cams for each of the foregoing. Thus, inline DOHC engines have two cams, and V-engines have four cams. Usually, DOHC are used on engines with four or more valves per cylinder, because a single camshaft cannot fit enough lobes to actuate all of the required valves.
The cam, with attached lobe(s), typically actuates a pivoted rocker arm to push down on the corresponding valves, which “opens” the valves to allow air and fuel into the cylinder. To close the valves, at least two main approaches have been used: desmodromic, and non-desmodromic (such as springs).
For non-desmodromic valves, springs typically are used to return the valves to their closed position. It is generally desirable that the springs are very strong because at high engine speeds, the valves are pushed down very quickly, and it is the springs that keep the valves in contact with the rocker arms. If the springs were not strong enough, the valves might come away from the rocker arms and snap back. This is an undesirable situation that would result in extra wear on the cams and rocker arms, and might even cause catastrophic failure such as if the valves come into contact with the pistons.
Push-rod type engines typically are made with non-desmodromic valves, with a camshaft located in the sump near the crankshaft. Most commonly the camshaft assembly includes cam followers (commonly called “lifters”) that push on tubular rods (“push rods”). The push rods push on pivoted rocker arms, which push the valve open. This “push rod” engine approach has more moving parts, and also causes more timing lag between the cam's activation of the valve and the valve's subsequent motion. A gear set, timing belt or timing chain links the crankshaft to the camshaft, so that the valves are in sync with the pistons. All of these methods of opening and closing the poppet valves in these push-rod engines require a spring (or similar action from, for example, a nitrogen/air bag) to close the valve.
Numerous problems can occur in systems that rely on springs or air bags to actuate a valve. Such problems include that valve springs in a conventional valve system are prone to harmonic forces that can cause the valves to bounce off the valve seat resulting in inadequate valve sealing and loss of engine power. Another “problem” in spring valve systems is that the high valve spring forces required in high-speed engines also mean that power is consumed to open the valves against these forces, resulting in less net power output. Accordingly, to obtain good results with a spring system, it is necessary to find a compromise between heavier spring loading required to turn at higher RPM while preventing valve bounce, and lighter spring loading to reduce the work required to open the valves against the spring loading.
In this regard, because valve springs must return the valves to their seats (sealing position), the higher the RPM the greater the kinetic force the springs must overcome, necessitating ever increasing pressures from the springs. In conventional valve system design, the design places fatigue stresses on the spring materials, resulting in failures. In the “push rod” engine design mentioned above, the springs must return to the closed position not only the valve, but also the rocker arm, push rod, and lifter. This can require spring forces at maximum opening of over 900 lbs. per spring. As such, valve spring failures are the most common failures in racing engines.
Rather than springs or air bags, desmodromic valve systems use extra cam lobes on the camshaft, with rocker arms activated by those cams that close the valves. The cams thus provide total control of the opening and closing action of the valves, rather than relying on separate spring elements for part of the valve action.
Desmodromic or spring-less valve actuation systems can reduce or eliminate the problems discussed above, and can provide control/smoothness, and consequently decreased power losses at low RPM, and reliability, without the loss of valve control at higher RPM. A few racing engines use a desmodromic valve system such as mentioned above, in which separate cam lobes control the opening and closing of the valve. Probably, the most famous were the Mercedes racing engines of the 1950s, including the legendary “Gullwing 300 SL” and today's Ducati motorcycles. These desmodromic systems used overhead camshaft designs to minimize components, weight, and space.
Desmodromic opening and closing of the valves further enhances performance by allowing cam designs of higher opening lift (the intake valve is opened to a “higher” position, so that it protrudes further into the cylinder), since they eliminate the limit of valve spring coil bind (when a coil spring(s) is compressed to the point the individual coils in the spring make contact with each other). This higher opening lift can result in greater volumetric efficiency for the engine, i.e., more air and fuel enters the cylinder for combustion, resulting in greater power output.
Another advantage of a desmodromic or spring-less valve system is that it eliminates a condition commonly referred to as “valve float”, wherein the valve is not following the camshaft lobe's shape, and may come into contact with the piston or valves. Desmodromic systems likewise eliminate concerns about coil bind, bounce, and harmonics. Desmodromic cam designs can accelerate the valve opening faster, hold the valve open for a longer duration of crankshaft rotation, and close the valve faster without fear that the closing valve may be contacted by the piston. These new design parameters result in more power output from better volumetric efficiency and cylinder sealing.
Accordingly, valve control in “push rod” type engines could be improved by eliminating the use of a return spring mechanism.
Improvements or modifications involving the contact between the valve rocker arm and the end of the valve of known valve systems as a means to improving various engine characteristics has been the subject of numerous patents. Several of these solutions arguably provide the “push-pull” connection necessary for desmodromic type valve action, but the designs require that unique valves be supplied. Because the valve system of the present invention is primarily intended for current and past production engines of the “push rod” design where the valves are of a commercial design, such unique valve designs or configuration are not necessary.
There exist numerous patents for desmodromic valve systems, but few have been mass-produced because of their complexity and critical tolerances. For example, U.S. Pat. Nos. 5,732,670 and 6,109,226 (both issued to Mote, Sr.) are directed to subject matter relating to “push rod” type engines, with the '226 patent describing replacing the more conventional push rods, lifters, and cam with an overhead cam assembly incorporating the geared rocker arrangement of the '670 patent. Among other things, the Mote technology apparently purports to result in a “push rod” engine with desmodromic valves. The Mote technology apparently has several shortcomings, however, in that it teaches to use gears as a linkage to the valves, it has no reversing pivot rocker, and it uses two lifters within a single bore.